Alderweireld has spent the last six years in North London, having joined from Atletico Madrid in the summer of 2015.
The 32-year-old has been instrumental in Tottenham’s rise into a major English and European force in the last few seasons, taking in 236 appearances in all competitions.
Al-Duhail will part with £13m for the former Ajax Amsterdam centre back’s services. He had two more years on his Tottenham contract.
Tottenham confirmed Alderweireld’s departure with a statement that read: "We have reached an agreement with Al-Duhail SC in Qatar for the transfer of Toby Alderweireld"
"We wish Toby the very best for the future."
Alderweireld has said his goodbyes in a social media post that reads: "Six years, two homes, a Champions League final and so many memories. You made me feel a part of your club the moment I walked out at White Hart Lane and roared us on, no matter what, into our new, special, stadium.
"Thank you first to the fans and then to my team-mates, and every incredible member of the Spurs staff, backroom and beyond.
"This club will always be in mine and my family's heart. Love Toby."